Well, for one, you can't post on bigger subs with your brand new account. You need to have some karma and you get it by posting and commenting with your account.
Subs (or subreddits) are like subforums where you post your links. They are divided according to the subjects (funny, porn, news or even bitcoin
https://www.reddit.com/r/Bitcoin/)
Some subs limit the posting ability with age of your account and limit the posters to only older than 1 or 2 years.
All those restrictions are made to reduce the spam, because it became ridiculous at one point on Reddit.
Spammers love Reddit because you can get a ton of traffic if you post to the right sub at the right time with the right content. And if you hit the front page you can count on 100 000 visitors or more (if your server can handle it).
That's it in a nutshell.
